1. Field of the Invention
The present invention relates to a method for the production of shaped foamed-glass articles having essentially uniform cellulation and useful as insulators against thermal and sound transmission and as light weight building materials such as pipe insulation.
It is well known that cellular or porous materials comprising a solid or fibrous material containing small interstices or cells filled with partially or completely entrapped air or other gaseous medium possess exceptional value as insulating media against the transmission of heat and sound. Asbestos (a carcinogen), cellulose fibers, and cellular material such as cork and organic polymer foams are representative of prior art efforts to produce insulative bodies. However, none of these materials is entirely satisfactory, asbestos being fairly expensive, lacking in mechanical strength, and having the additional disadvantage that it tends to compress during use, thereby losing at least a portion of its insulative value, and the organic materials lacking in resistance to heat, fire and chemicals. Consequently, foam glass materials have become of increasing importance as thermal insulators due to the inherent properties of low thermal conductivity and high resistance to chemical corrosion and fire. However, due to high cost of production, as well as unsatisfactory methods of production, these foam glass materials have not been generally adopted by the industry, which has continued to use the more common asbestos, cork, polymer foam or cellulose fiber materials, notwithstanding the failure of such materials to possess the required properties.
One reason for industry's decision to continue to utilize these less than optimum materials is the cost factor. Production methods wherein foamed glass blanks are formed and then machined to the appropriate shape result in such expensive labor costs and such a high waste factor as to render these methods commercially unacceptable.
Efforts to extrude shaped foamed-glass articles have not met with commercial acceptance. Production of shaped foamed-glass articles wherein an unfoamed molten glass is extruded into an area of reduced pressure, whereupon foaming occurs, must deal with the following problems. The presence of entrained gas bubbles, under pressure, in the melt prior to extrusion, results in a foam product having such non-uniform cellulation as to make it unsatisfactory. In order to remove such entrained gases in a reasonable period of time, the viscosity of the molten glass must be reduced to a range of below 10.sup.3 poise. However, one cannot foam the glass at such a viscosity range, the cellulation at this viscosity range being uncontrollable. To achieve satisfactory cellulation, a viscosity in the range of 10.sup.5 to 10.sup.7 poise is required. A temperature reduction of about 200.degree.-300.degree. C. is required to increase the viscosity to the range of 10.sup.5 to 10.sup.7 poise. An additional factor which must be taken into consideration is that foaming must not occur prior to extrusion. Foaming can be controlled by application of a pressure sufficient to inhibit gas formation of the foaming agent.
Therefore, in order to prepare shaped foamed-glass articles by extruding foamable molten glass compositions, containing a foaming agent, into an area of reduced pressure, whereby foaming occurs, it is necessary to (1) remove all entrained gas bubbles prior to extrusion, (2) completely inhibit gas formation prior to extrusion and, (3) extrude the molten glass into an area of reduced pressure, where foaming occurs. It is also essential that the particular foaming agent in question actually produce a gas at the extrusion temperature having a vapor pressure greater than one atmosphere, preferably more than two atmospheres (30 psia). Otherwise, if the vapor pressure of the gas produced by the foaming agent does not exceed the atmospheric pressure of the area into which the molten glass is extruded, no foaming will occur.